The present invention relates to a process for the thermal treatment of high strength aluminum alloys of the Al-Zn-Mg-Cu type, such alloys typically being referred to as the 7000 Series and which contain more than 0.05% by weight of copper. The process is particularly useful for treatment of thick products, that is, products which have been wrought by rolling, forging, extrusion, die stamping, etc., and including articles such as bars, billets, sheet billets, thick sheets and other articles having at least one part which is thicker than 15 mm. The conventional treatments for hardening alloys of this type comprise the following sequential steps:
(1) solution heat treatment; PA1 (2) quenching; and PA1 (3) tempering PA1 (3a) pre-tempering in the zone from 100.degree. to 150.degree. C. for a period lasting from 5 minutes to 24 hours; PA1 (3b) intermediate tempering at a higher temperature; and, PA1 (3c) final tempering for 2 to 48 hours at between 100.degree. and 160.degree. C.
possibly with cold plastic deformation of 1 to 5% between the second and third steps for stress-relieving the products in the crude state of quenching. This plastic deformation is generally obtained by controlled traction of rolled or extruded flat products (known also as the TXX51 state) or compression of forged or die-stamped products (known as the TXX52 state). The tempering treatment leading to the highest mechanical tensile characteristics generally involves increasing the temperature to below 140.degree. C., keeping the temperature isothermal and cooling. Such a state is generally known as T6, T651 or T652 depending upon the nature of the plastic deformation after the quenching treatment and is not generally utilized for thick products since poor resistance to stress corrosion is usually encountered in the short transverse direction and to exfoliant corrosion.
The conventional treatment for tempering thick products generally involves a first isothermal step at a temperature below 140.degree. C. followed by a second isothermal step at a temperature above 150.degree. C. and then cooling, each step often being preceded by a slow rise in temperature. It is intended to impart a high resistance to stress corrosion in the short transverse direction, but results in a very substantial reduction in mechanical characteristics with respect to the T6 or T651/T652 state. This state is known by the skilled artisan as the T73 (or T7351, T7352, depending upon the nature of the cold working after quenching) for the 7075, 7175 and 7475 alloys and T736 (or T73651, T73652) for the 7050 alloy. In the conventional treatment a tempering step is intended to impart to rolled products made of 7075 (or 7175, 7475) alloy mechanical tensile characteristics and a resistance to stress corrosion which fall between those of the T6 (or T651) states and T 73 (or T7351) states with a high resistance to exfoliating corrosion. This tempering treatment is similar to the T73 (or T7351) tempering treatment, but the periods of treatment are generally shorter. This state is known as T76 (or T7651) by the skilled artisan and is applied in particular to thin or medium sheets.
Resistance to stress corrosion is generally evaluated on samples which have been cut out in the short transverse direction by means of alternate immersion and emersion tests (10 minutes-50 minutes) in a reagent containing 3.5% NaCl in accordance with American Standard ASTM G44-75 (Standard Recommended Practice for Alternate Immersion Stress corrosion Testing in 3.5% sodium chloride solution). The resistance to exfoliating corrosion is evaluated by the EXCO test in accordance with American Standard ASTM G34-72 (Standard Method of test for Exfoliation corrosion Susceptibility in 7XXX Series Copper containing Aluminum Alloys). However, it is possible to simultaneously obtain these two properties which are apparently contradictory (high mechanical characteristics and high resistance to stress corrosion) if a tempering step comprises the following treatment made:
A treatment of this type is described in French Pat. No. 2,249,176 and comprises a short period of intermediate isothermal tempering performed in practice by immersing very small products (cross-section of 1 cm.sup.2) in a metallic bath such as Wood's metal. Now it is known that the immersion of aluminum alloys in a medium of this type can lead to very severe intergranular brittleness of the alloys. Moreover, the heating method employed can rarely be considered adequate in view of the difficulties in employing it due, in particular, to the high density of the bath, particularly in the case of large products, for example thick sheets. Finally, although the treatment conditions described are valid for any small articles, they cannot be applied industrially since they do not take into consideration the thickness of the articles. Now it is obvious that the real thermal cycle undergone by the article will be very different, depending upon the thickness of the article.
It is therefore an object of the invention to provide a thermal treatment process for relatively thick products formed of copper-containing aluminum alloys to improve the mechanical strength and stress corrosion resistance thereof.
Further objects and advantages will become more readily apparent in light of the following detailed description of the preferred embodiments.